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We have some sensitive equipment that just does not like choppy square wave that some VFDs output. Curious if output caps can help smooth this curve to get more sinusoidal output or how else one might go about getting simulated wave output from a pulsed wave form?
LOL. can I get some hand holding too? Google is about as reliable as my 20yo golden retriever... Are you saying it is common knowledge to get sine wave from PWM output?
I haven't stumbled across a harmonic line filter that allows us to make accurate measurements of PWM output voltages from a VFD yet. The one (and only) somebody else tried left them wondering if the phase difference between the voltage and amperage signals was accurate.
I've had good luck using a simple set of three shunt resistors and six RC filters. The filter caps are connected to chassis ground (not to any one of the other five signals). I think the current setup uses 50:1 voltage dividers and 0.03Ω shunt resistors connected to 75KΩ/0.01µF RC filters with ≈500VAC safety caps connected to fully isolated 5B modules.
BTW, it works but currently trips GFCI's with the measurement box attached.
Not sure how to specify the extended frequency response.
Are you trying to measure voltage, current or power? What, actually are you dealing with? Load wise, duty cycle wise, frequency wise etc?
I've used some of the ION series meters: **broken link removed**, but I cannot remember the model. Nice piece of equipment. We used one of their 3 phase meters to measure a single phase, phase angle fired signal into a transformer with very low voltages 0-32V with no issues.
I was misled by your question. I thought you had issues with interference, not measuring of basic quantities. If your trying to measure power, use an integrated solution. Not the RMS value of V multiplied by the RMS value of I. It may not work. There stuff is basically qualified to be a utility meter.
We have some sensitive equipment that just does not like choppy square wave that some VFDs output. Curious if output caps can help smooth this curve to get more sinusoidal output or how else one might go about getting simulated wave output from a pulsed wave form?
I am missing something here. A good VFD should output a sine wave and not a square wave or modified square wave. PWM is a different animal all together.
Currently as to a VS drive I am using a few of these for one test. They perform flawlessy from frequencies of sub 1 Hz. to 60 Hz. However, in the best interest of motor protection in general I use these units which I call the chokes from hell with a few caps tossed in. Sort of a pi filter design. They use (in my case) 3 EA. (3 phase units) 0.7 mH chokes rated for 80 amps and some caps in a pi configuration. I forget the value of the caps we have in them.
Anyway, depending on what you have and what you need if you want a turn key solution MTE makes a full line of filters or roll your own.
We're measuring Volts, Amps, Power, and Frequency being supplied to a 5hp motor driven by a Delta VFD under zero load, normal load, and locked rotor conditions.
We want to make sure any filters used do not delay, attenuate, or amplify any one of these parameters differently than any of the others as they are being used to characterize and subsequently model the motor under test. This is where the inductance of the scavanged reactor investigated by my predecessor appeared to be shifting a couple of these measurements relative to one another. Thus far our data seems to correspond to the theoretical induction motor behavior within 5% or so. Unfortunately this doesn't involve a high volume product and this bit of analysis understandably has a pretty low priority. That is, I've only done a couple of motors at 20, 50, 60, and 90 Hz.
Measuring Amps and Frequency by themselves is not problem but as soon as Voltage and Power/Power Factor are introduced the 16 KHz switching frequency of the drive finds its way into everything. The three shunts and three dividers are arranged in a Delta configuration and the six RC filters have an impedency of about 150 KΩ at 2.0 KHz. and 85 KΩ at 16 KHz. The outputs of the RC filters are connected to 10 KHz 5B40-series modules with sampling being done at 4 KHz. In spite of the trouble we've taken, a small amount of the voltage signal still appears on the amperage inputs. Additional 1st order filtering in software and a little curve fitting to match the readings from some old analog meters we still keep in the calibration program has gotten us what we're looking for, reasonable estimates of torque and efficiency from 10Hz to 110 Hz.
BTW, given my work environment, I either have to do this cheaply or justify the cost of another reactor with an inability to get what we need without it.
Am I going too far to do this cheaply? Can the appropriate reactor between the drive and the motor potentially get me another 3-5% accuracy?
Of course I'd then need to justify the need for another 3-5% accuracy.
In a nutshell, we are wanting to come up with a filter array that will improve output quality near true sine of a VFD to use the output side for more sensitive electronics components. Constantly running into machine situations where 3ph is not available, we want to operate a VFD at 60hz and use for conversion purposes. A motor would not really care but those harmonics can really create issues for other driven electronics as we have experienced in the past.
Yes, we would need to look at the voltage, PF, phase angle, and over all wave form.
The problem with filters is they are signle frequency in nature ( unless you can get a monkey to contstatly tweak the cutoff frequency in step with the input ) The best way I've found to make a sine wave is to first convert the rectangular wave to a triangular one, then either use a breakpoint shaper or a logarithmic shaper.
Being on the ignorant side of this, I was not previously aware that VFDs actually output a large amount of pulses that "build up" the wave pattern to simulate since wave output. I had my head buried in DC drives where the output is pure square wave and thought that is what they were doing but it sort of makes sense to help protect an AC motor. I am honestly still a little dumb struck in how you can vary the output voltage in stages to "build up" the wave voltage ramp since the IGBT only knows 2 things, on and off. The output simply dumps out DC buss voltage so in my mind, I am trying to understand how you get a ramped voltage output in the first place.
To further on filtering, It seems that because if the high frequency switching, this is what causes the harmonics on the line that give some false shape to the wave pattern. It sounds like reactors are the common medicine for much of this but I am still trying to determine how perfect things really need to be anyway. I thought people were using phase to phase caps on the output to change the rise and fall of a square wave to simulate sine wave. I might need to reschool myself here and that might only effectively "shift" the even rather than "shape" it..
When you integrate variable width pulses at a much higher frequency than the fundamental, you change the effective amplitude. "Integrate" basically means applying a low pas filter to the resultant waveform.
About the best you can hope to do without a LOT of expense is to slow the edges down with an LC filter. This is often enough to get around the sensitivity issues i.e. Fast edges = glitches. There is also a problem that can occur if a simple bridge rectifier is used in the equipment, the peak voltage of a PWM waveform is very different to that of a sine wave. Motors do the averaging because of their inductance and inertia, but electronics might not.
I assumed it was messing with measurement equipment.
The Delta VFD's I work with don't work like power supplies. I might be able to reconfigure all of the control parameters intended to model the motor internally and disable those intended to detect motor faults but I still doubt it would tolerate loads that differ substantially from a motor.
We use an Elgar Smartwave to do this when we need to.
We can set up the outputs as one phase in series, two phase, or three phase and simulate arbitrary waveform disturbances. It's quite cool but was quite pricey and is total overkill for our application. The idiot who got it approved doesn't even know how to use it. It sat unused for 18 months because he couldn't figure out how to properly wire it up and configure it. I finally got sick of looking at it and had an application I could use it for so I read and reread the haphazardly organized manual until I worked out where he made his mistakes.
The output is clean as a whistle and has yet to cause a problem with anything we've plugged into it. Check out their website. They do have cheaper options.
The aforementioned idiot managed to convince management that it couldn't be his product that was faulty, it had to be something in the power lines. He had to have this fancy schmancy AC power source to figure out what sort of power line distubances were causing his product to fail. I'll bet you can guess where the problem was. Someone else found it before this non-returnable AC source had even arrived.
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